The present invention relates to a circuit for synchronizing a plurality of power units with one another, and more, particularly to the type used in power stations for telecommunication systems.
Usually the latter are constituted by a plurality of power units (such as inverters, etc.) connected in parallel with one another. The parallel connection is used when the power station has to generate an amount of power that a single power unit alone could not supply and therefore, the required power is supplied by a plurality of power units connected in parallel one with another. Besides the number of units needed to provide the operating power, at least one additional unit is usually required in order to have a hot, active reserve in case one of the operative units fails. In almost all applications there is the problem that the units must be synchronized with one another and/or with the electrical power distribution line (i.e: the mains).
In particular, synchronizing operations are necessary when the units are DC-DC converters because, in case of a phase difference among the single units, low frequency interferences which lap oer the useful signal, may be registered.
On the other hand, when the power units are inverters, the synchronizing means are absolutely necessary, either to avoid damages described below, or to overcome the technical troubles which may arise from the switching power station-power distribution line.
It is useful to point out that the inverters are constituted by units adapted to receive a direct current on the input side and to deliver an output of alternating current having the same characteristics of the power distribution line.
The above mentioned switching must be carried out however with phase continuity, otherwise the single units will be pulled in to the mains at differentiated speeds and cause a phase irregularity from the single units which compose the power station.
The inverters of the latest generation are characterized by the fact that they have an output impedance that is sensibly low in order to approximate the characteristics of an ideal voltage generator. This characteristic implies a perfect synchronism among the inverters of a same station; because if one of the units is not in phase with the remaining units, the power delivered by the remaining units is transferred into the unit which is out of phase causing possible damage to it.
In accordance with well known solutions, the synchronization troubles mentioned above may be solved by sending to a common branch point the clock signals generated by the single inverters. A reference clock signal is thus created at the branch point that has a frequency equal to the arithmetical average of the inverters' frequencies. The devices are synchronized with the reference clock signal.
This system however presents the inconvenience that the pulling in field for the different oscillators must be limited in order to maintain the average frequency within acceptable values. Furthermore, the connection or disconnection of an inverter causes a transient frequency on the inverters actually operating.
Other solutions provide for a generation of a master clock signal through a central oscillator on which the single inverters are synchronized. This solution does not present the inconvenience of the connection transient described above, (because the inverter is connected once the synchronization has been adjusted), but on the other hand the following troubles may arise:
it maybe necessary to have a slave oscillator, operating in phase with the master oscillator in order to carry out the switching with phase continuity;
it introduces limits to the system's reliability due to the circuits which provide for the master-slave switching;
the cost of the structure described above (master oscillator, slave oscillator, switching circuits) constitutes a large part of the cost for a station when the station is only composed of two inverters.
The invention, as claimed, is intended to solve the problem of realizing a circuit without having the inconveniences mentioned above and at the same time it is suitable for switching a load to the electrical power distribution line without creating phase differences among the single inverters of the power station. For this purpose and according to the present invention, the circuit provides the connection of the respective synchronizing unit to each power unit. Each of the units has been designed to receive an activation signal at its input, to be connected to the other units by means of a first and a second bus, and to deliver to the output, the synchronizing clock signal for the relative power unit. The first synchronizing unit which receives the activation signal is characterized as the master unit. This master unit applies a predetermined polarity to the first bus and delivering a clock signal to the second bus in order to synchronize the other units which are characterized as slave units as a response to the detection of the polarity.
When the master unit receives a disconnection instruction at its input, it modifies the polarity on the first bus and qualifies the synchronizing unit, whose output clock signal has its phase shift anticipated in comparison to that of the other units
The circuit as claimed in the present invention is characterized by the fact that each slave unit constitutes a standby unit readily available to replace the master unit which enable the following advantages:
possibility of large pulling-in fields;
high frequency stability for the master clock;
no centralized circuits on which the system's reliability depends;
no cost increase when the station is only constituted by two power units.